Building block for a toy building set



FIG. 1 is a top plan view of my ornamental design for a building block for a toy building set;

FIG. 2 is a top, right perspective view thereof;

FIG. 3 is a front side view thereof;

FIG. 4 is a left or right side view thereof;

FIG. 5 is a bottom plan view thereof; and,

FIG. 6 is a bottom, right perspective view thereof. 

CLAIM The ornamental design for a building block for a toy building set, as shown and described. 